Cracking process



Apfil 27, 1937. g, YDN R 2,078,899

i CRACKING PROCESS Filed July 23, 1932 Patented Apr. 27, 1937 UNITED SPATENT orFrcE CRACKING PROCESS Harold Sydnor', Westfield, N. J.,assignor to Standard Oil Development Company, a corporation of DelawareApplication July 23, 1932, Serial No. 624,256

1 Claim.

. ly in section, of the apparatus used for carrying out the invention.

The efforts of the petroleum refining art have lately been directedtothe production of good knock rating gasoline. This purpose can beachieved in the so-called tube and tank cracking process in which theoil to be crackedds passed through heating coils into a so-calleddigesting or soaking drum and the cracked prod not is then separatedinto its constituents such as tar, cycle stock, naphtha and gas. When itis required to produce a good knock rating gasoline in such a process itis necessary to maintain a higher cracking temperature than was formerlyusual. ,A serious limitation to such high ternperature tube and tankprocesses at the present time is the tendency of the release lines and 3I valves immediately adjacent to the drum tonclog due to theaccumulation of coke which terminates the run sooner than wouldotherwise be necessary. I have found ficulty can be eliminated bytransferring the cracked material from the soaking drum into a separatorheld substantially under the same pressure as the soaking drum and byintroducing fresh feed into the separator to cool down its contents andcheck the further cracking. By thus that this coking difmaintaining fullsoaker pressure on the products leaving the soaking drum until they arecooled by direct contact with the incoming fresh feed, the coking of thelines, adjacent to the soaking drum becomes eliminated.

Among the several other objects and advantages of the present inventionthe following are mentioned.

1. -A hi her than the usual preheatof the'oil. entering the heatingtubes is obtained since the necessity of recirculating cold tar to thebottom of the separator for cooling becomes eliminated and the excessheat content of the cracked efliuent from the soaking drum is used forpreheating the fresh feed. The advantage of carrying out the directcontact of the incoming feed with the cracked products before release ofpres;- sure becomes greater ture is increased since the amount of heatto be dissipated betweenpthe soaking drum and the separator ofa unitoperating with a low pressure as the coil outlet temperaseparatorincreases with the cracking temperature. High preheat is essential tothe development of a high temperature, high pressure cracking, since itmeans less heating surface to reach the, cracking temperature at a givenfeed rate, which in turn means less tubes and less pressure drop throughthe heating tubes.

2. It is Well known that naphthas produced by high temperature crackingare unstable and contain a high percentage of gum forming constituentswhich have to beremoved by acid treatment. It has been applicantsexperience that the stability of the produced naphtha against gumformation increases when the high pressure separator of the presentinvention is used. Due to increased stability of the gasoline, thetreatment of the same with sulfuric acid for the removal of gum formingconstituents may be eliminated and the gasoline. finished by simple claytreatment as will be described hereinafter. H

3. I also found that the use of the high pressure separator in acracking process results in the formation of a gasoline having greaterresistance to detonation with increasing engine jacket temperature whenused as armotor fuel. The resistance to detonation is a function of thechemical constitution of the gasoline and is expressed in octane numbersas well known in the art. The octane number varies with the conditionsunder which the engine is run and more specifically with the enginejacket temperature, there being a drop in the octane number as thistemperature increases. I found that the drop in the octane number withincreasing engine jacket temperature will be less when the gasoline isprepared bya cracking process using a high pressure separator accordingto the present invention. l. In onemodification of the present processthe fresh feed is introduced at the top of the separator while thecracked product is introduced at or near the bottom of the same so thatthe hot vapors and the cooler liquid pass in countercurrent to eachother. It will be seen that a clean and fresh distillate feed stock isthus obtained for the passage through the heating tubes of the furnaceand any diificulty due to deposition of coke in the heating tubes onaccountof unclean charging stock is eliminated.

v Referring now to the drawing the same will be described in conjunctionwith the method of op- .erating the process.

The feed stock consisting of fresh feed distillate and cycle stock ispumped from accumulator I through line 2 and preheater 3 to theheatingcoil 4 arranged in furnace 5. The oil is heated in the coil to acracking temperature and then discharged into the digester or soakingdrum 6 in which the desired degree of cracking takes place. The hotcracked effluent leaving the drum by line 1 is then transferred into thehigh pressure separator 8 in which it is cooled by partially preheatedfresh feed in order to check the further progress of the cracking and tosimultaneously vaporize the vaporizable constituents of the latter.Fresh feed from tank 9 is pumped through heat exchange coil I0, heatexchanger H and line l2 to the top plate of the separator 8. Valve I3and line 14 allow the passage of a small part of the preheated freshfeed to the bottom of the soaking drum mainly for the purpose ofpreventing the coking of line 1. Release valve l5 allows the maintenanceof a high pressure on coil 4, drum 6 and separator 8. It will beunderstood that there is considerable pressure drop in the elongatedcoil 4 due to the passage of the oil therethrough at a high velocity butthe pressure in drum 6 and separator 8 will be substantially the samesince the pressure drop in the short line I is comparatively small. Thepreheated fresh feed is preferably introduced at the top plate ofseparator 8 which is provided with discs and doughnuts or equivalentseparating and fractionating means. The introduced fresh feed coolsthehot cracked effluent and checks thereby the further cracking, at thesame time it is vaporized so that it leaves the separator in admixturewith the cracked vapors through vapor line 16.

The liquid level control I1 connected with the release valve l8 and thebottoms line H! allows the maintenance of a certain low liquid level inthe separator and the removal of the unvaporized bottoms therefrom. Thebottoms consists of tarry matter from the cracking and from the freshfeed in case an unclean charging stock is used as fresh feed. The vaporsleaving through line [6 will be substantially free of any heavy tarrymatter and are subjected to partial condensation and fractionation inheat exchangers 3, l I and fractionating tower 20. The latter isprovided with fractionating means such as bell cap plates and a heatexchange coil It at the top of the tower. The liquid which condenses inthe tower 20 is collected in pan 2| from which it flows through line 22into the accumulator l. Similarly the condensed oil from heat exchangers3 and II flows through 23 into the accumulator I to provide a cleancharging stock for the cracking coil. There is connected with the lowerend of the fractionating tower 20 a low pressure separator 24 into whichthe bottoms leaving the high pressure separator 8 through line l9 aredischarged. Here the lower boiling constituents contained in the highpressure separator bottoms evaporate due to the release of the pressureand pass through center opening of pan 2 I into the fractionating tower.Unvaporized tar is removed through line 25 and tar cooler 26 to astorage tank (not shown). The uncondensed oil vapors leave the top ofthe fractionating tower 20 through vapor line 21, are condensed in finalcondenser 28 and are obtained as naphtha after the separation andremoval of the permanent gases through line 29. Due to the stability ofthe naphtha obtained in my process the sulfuric acid treatment may beeliminated and the stock may be finished by a clay treatment in a claytreating apparatus diagrammatically shown at 30 whereby a treatednaphtha is ob-' tained.

The operating conditions may be varied within a considerable range. Thepressure maintained on the soaking drum and the high pressure separatoris preferably higher than about lbs. per square inch and is usuallyaround 1,000 lbs. per square inch although pressures higher than thatmay also be employed. The temperature to which the oil is heated in coil4 may also vary within a broad range. It is usually higher than 800 F.and is preferably between 900 and l000 F.

The following example will illustrate my invention.

A 31 A. P. I. West Texas gas oil was cracked according to the presentinvention, using a mixed feed rate to the cracking coil of 25,000gallons per hour of which 50% was fresh feed. The conditions of thecracking were the following:

F. Coil outlet temperature 950 Soaking drum temperature 900 Highpressure separator temperature 825 Coil inlet temperature 775 Thepressures were the following:

Lbs. per sq. inch Hot feed pump 1,000 Soaker 750 High pressure separator7 50 Bubble tower 100 Low pressure separator 100 The following yieldswere obtained:

Percent Distillate of 400 F. end point by volume" 54.6 Fuel oil of 9 A.P. I. gravity by volume 31.6

Gas by weight 15.4

ing zone to effect the required amount of crack- 1 ing, passingsubstantially all of the cracked effluent to an enlarged high pressureseparating zone, maintaining substantially the same high pressure in thedigesting and separating zones,

introducing fresh feed oil below cracking tem- 5' perature to theprocess, passing a part thereof into said digesting zone in proximity tothe point of withdrawal of the heated oil therefrom, passing anotherpart of said fresh feed oil into the separating zone, contacting saidlast mentioned part of said fresh feed oil with cracked efiluent fromthe digesting zone to effect vaporization of the fresh oil at crackingtemperature therein, removing oil vapor from the separating zone andfractionating the same under lower pressure than that in the separatingzone to obtain the said relatively clean distillate charging stock andnaphtha, returning clean distillate charging stock thus obtained to saidheating zone as charge oil therefor, separately removing unvaporizedbottoms from the separating zone and eliminating said bottoms from thesystem without subjecting them to thermal conversion.

HAROLD SYDNOR.

